The understanding of basic cellular processes has been greatly facilitated through investigation of the behaviour of mutant forms. In a similar way the mechanisms of genetic recombination may be clarified by a study of strains which are known to show inherited differences in recombination behaviour at meiosis. The haploid fungus Neurospora crassa is particularly well suited to such an investigation since recombination frequency heterogeneity has been extensively reported in that organism, and the differences are believed to be, to a large extent, under genetic control. Strains showing recombination frequency heterogeneity over a marked genetic region have been extensively analysed in the present work and the mode of action of the factors controlling recombination frequency has been investigated by combining differing strains in heterokaryons. / Thesis / Doctor of Philosophy (PhD)